In an optical communication system which employs optical fibers for transmitting optical signals, it is known that optical signals disperse within optical fibers are effected by disturbed signal waveforms, resulting in limited transmission distances. The dispersion is a phenomenon attributable to a difference in the diffractive index from one wavelength to another within an optical fiber that cause signals to arrive at different timings. Since an optical signal generally has a predetermined spectral width, a long transmission distance would cause its signal wave to extend on a time axis due to the dispersion, resulting in a failure to correctly receive information on the receiver side.
In basic networks, access networks and the like, light sources in a 1.55 μm band are used because losses are generally minimized in this band within an optical fiber. An optical signal within this wavelength band receives a dispersion value of approximately 17 ps/nm/km within an optical fiber. Therefore, the influence of the dispersion can be alleviated during optical transmissions if the dispersive properties of optical fibers are shifted such that a dispersion value is minimized for optical signals within the 1.55 μm wavelength band. An optical fiber, the dispersive properties of which are shifted from a predetermined wavelength band, is called a “dispersion shift fiber” (DFS).
However, since the dispersion shift fiber has a complicated refractive index distribution in a cross-sectional direction as compared with an ordinary optical fiber, the dispersive properties are directly affected by distorted cores due to routing during the installation of optical fibers, so that dispersion values vary depending on positions of optical fibers. Also, the dispersion values largely fluctuate due to changes in the ambient temperature. For these reasons, in conventional optical communication systems which employ dispersion shift fibers, overall transmission paths are designed in detail in a distributive strategy, but an increase in the distance and capacity of the transmission path has been impeded by higher cost of the dispersion shift fibers which are manufactured based on the result of this design.
Generally, within an optical fiber, the product of the square of a transmission speed and a dispersion value of an optical signal is constant. Therefore, if an optical signal has a spectral width, a transmittable distance is reduced in reciprocal proportion to the square of the speed.
For example, when a directly modulated semiconductor laser is used as a light source, the dispersion exerts a large influence because of large fluctuations in the wavelength of the light source. On the other hand, when an externally modulated semiconductor laser is used as a light source, fluctuations in the wavelength can be limited by adjusting an external modulator of Mach-Zehnder type, so that a transmission speed can be realized in a range of 10 to 40 Gb/s. However, even in this situation, the influence of the dispersion cannot be eliminated.
Therefore, optical communication systems generally employ an approach of increasing a transmission distance by reducing the influence exerted by the dispersion with the provision of a dispersion compensator disposed on the light source side or light receiver side for giving a dispersion which has properties opposite to those of dispersion given by optical signals within optical fibers.
As an example of providing such a dispersion compensator, a configuration which comprises a dispersion compensator in a transmission unit (light source side) is disclosed in Patent Document 1 (JP-10-242910-A). Also, as a specific configuration of a dispersion compensator, Patent document 2 (JP-5-181028-A) and Patent document 3 (JP-5-323391-A) each describe a dispersion compensator of a fiber ring resonator type. Further, a dispersion compensator of a fiber grating resonator type described in Patent document 4 (JP-2000-252920-A), a dispersion compensator of a multiple reflection delay plate type described in Patent document 5 (Published Japanese Translation of PCT International Publication for Patent Application No. 2000-511655), and the like are known as well.
Since these dispersion compensators employ a large scale configuration in order to compensate for the dispersion, the problem of an increase in the cost of the optical communication system will occur. Nevertheless, these dispersion compensators are suitable for use in optical communication systems for long distance transmissions because they are capable of compensating for a large amount of dispersion (for example, 3000 ps/nm).
On the other hand, as an attempt to reduce the size of the dispersion compensator, Patent Document 6 (JP-6-160604-A), for example, describes a dispersion compensator which utilizes an etalon resonator (hereinafter called the “etalon type dispersion compensator”). In the following, this etalon type dispersion compensator will be briefly described.
The etalon resonator includes two partially transmitting mirrors (partially reflective layers) arranged with a predetermined spacing therebetween to form a reflection type resonator, wherein peaks appear in the transmission property of light at intervals of a predetermined frequency called “FSR” (Free Spectral range). FSR is a frequency determined by the spacing interposed between the arranged partially transmitting mirrors. For example, in a configuration comprising an optically transparent layer having a refractive index of 1.5, such as glass, and sandwiched by partially transmitting mirrors, FSR is calculated to be approximately 100 GHz when the spacing between the partially transmitting mirrors is 1 millimeter.
Incident light on the etalon resonator is transmitted through the resonator while repeating reflections between both end faces of the two partially transmitting mirrors. In this event, while wavelength light having more transparent components and wavelength light having less transparent components can be observed, any wavelength light reflects on the end faces at an equal rate, so that it is thought that wavelength light having more transparent components reciprocates a larger number of times and is therefore transmitted through the etalon resonator with a larger delay. In other words, the dependency of the transmission property on the wavelength means that a difference is produced in the delay time depending on the wavelength. The etalon resonator can be used as a light dispersion filter having large dispersive properties because its delay time largely varies in response to fluctuations in the wavelength. Among the dispersive properties of the etalon resonator, if part of the properties opposite to the dispersive properties of an optical fiber is utilized to give dispersion to an optical signal which enters into the optical fiber or an optical signal delivered after transmission in the optical fiber, it is possible to reduce the influence of the dispersion given by the optical fiber.
For the etalon type dispersion compensator, there is also known a reflection type configuration which completely reflects light on one end face. When one end face is made to be completely reflective, light which should otherwise exit through the end face again passes along the etalon back to the incident side, so that the incident light is emitted from the same plane as the incident plane without loss. The amount of dispersion experienced by the emitted light is the sum of the amount of dispersion of the light which would otherwise be reflected, and the amount of dispersion of the light which would otherwise exit through the end face.
Since the etalon type dispersion compensator provides a substantially equal transmission property at every predetermined frequency interval (FSR), it can be used as a dispersion compensator for simultaneously compensating optical signals on multiple channels for dispersion, as in an optical wavelength division multiplexing (WDM).
However, conventional etalon type dispersion compensators can merely accomplish a dispersion compensation value of approximately 20 ps/nm, and the light dispersion filter has a transmittance of lower than 100%, thus causing an increase in the loss of optical power.
On the other hand, Patent Document 7 (JP-2000-105313-A) proposes a method to increase a dispersion compensation value by using a plurality of resonator type filters and expanding the bandwidth. Patent Document 7 describes that the dispersive properties are improved by stacking three layers of resonator filters each including a dielectric multi-layer film having predetermined dispersive properties.
However, the configuration described in Patent Document 7 employs a reflection type structure having a light incident plane, which also serves as an exit plane, in order to extract 100% of optical power while compensating for the dispersion, so that a device (circulator) is required for changing the optical path of incident light or emitted light. This requirement results in a large configuration of the device on the light source side or light receiver side which has the dispersion compensator, thus making the configuration unsuitable for a reduction in size. Further, since only dispersion is compensated for within a predetermined band, this configuration is not suited for multi-channel applications such as the etalon type dispersion compensator described in Patent Document 6.
Thus, Non-Patent Document 1 (Moss Optical Fiber Conference, 2002 Institute preprint manuscripts, TuT2, p133, FIG. 1) describes an exemplary modification to the configuration described in Patent Document 7 for multi-channel applications. However, since the configuration described in Non-Patent Document 1 is a reflection type, i.e., having a light incident plane which also serves as an exit plane, similar to Patent Document 7, it requires a circulator and the like, and is therefore not suited for enabling size reduction. Further, an increased number of parts, required for a mechanism for varying the amount of dispersion, make the configuration complicated.
Patent Document 8 (Published U.S. patent application No. 2001-0021053) describes an exemplary configuration which is a similar configuration to Non-Patent Document 1 but does not employ a circulator. However, the configuration described in Patent Document 8 also has a problem similar to Non-Patent Document 1, and includes the difficulty of having an optical communication system and an optical system that have an increased size.
Problems of the conventional light dispersion filters described above may be summarized as follows:
A first problem is that the system configuration becomes large in an optical communication system which has a conventional dispersion compensator installed on the light source side or on receiver side.
This is because an optical path must be made sufficiently long in order to delay an optical signal for purposes of accomplishing a large dispersion compensation value because the dispersion compensation necessitates the creation of a delay circuit corresponding to a wavelength component.
A second problem is that optical power is lost without any benefit in the conventional dispersion compensators.
This is because a light coupling loss occurs when an optical signal is extracted from an optical fiber for performing the dispersion compensation and the optical signal is again returned into the optical fiber.
A third problem is that a dispersion compensation value is small in the conventional etalon type light dispersion filter.
This is because the conventional etalon type dispersion compensator cannot increase the dispersion compensation value due to a short path for delaying light.
A fourth problem is that the system scale is large due to a complicated optical system in the conventional optical communication system which comprises a reflection type dispersion compensator.
This is because parts for switching optical paths are required since the reflective dispersion compensator has a light incident plane which also serves as an exit plane.